Patent Document 1 has described a technique in which it is determined that polarization of a battery is eliminated when a predetermined time has elapsed since switching of an ignition key to OFF. Since charge and discharge of the battery is not performed after the switching of the ignition key to OFF, the polarisation of the battery can be changed to be eliminated. When it is determined that the polarization of the battery is eliminated, the current and the voltage of the battery are sampled, and the SOC (State Of Charge) of the battery is estimated, on the basis of the current and the voltage.
In the battery where the polarization occurs, the polarization may vary to reduce the accuracy in estimating the SOC. To address this, in the technique described in Patent Document 1, the SOC of the battery is estimated after the elimination of the polarization of the battery is determined, thereby improving the accuracy in estimating the SOC.